Production plants for manufacturing containers (such as beverage cans) can produce a very large number of containers, with sophisticated (multicolor) decoration thereon, in a relatively short amount of time. For instance, a conventional decorator in a container production plant can decorate 2,000 containers per minute. Container decorations have intrinsic value, as consumers tend to attach perceptions of quality of a product based upon the design on the container that holds the product.
Conventionally, there is a lack of robust inspection of exterior surfaces of containers at these container production plants. A known process for container inspection is tasking an operator at the plant with periodically pulling containers from a conveyor for visual inspection. For instance, every so often (e.g., every 15 minutes), the operator may be tasked with pulling a small number of containers off of the conveyor and visually inspecting the containers to ensure that the exterior surfaces of the containers are free of readily apparent defects (e.g., to ensure that proper colors are applied to the exterior surfaces of the containers, to ensure that the exterior surfaces of the containers are free of smears, etc.). Using this conventional approach, thousands of defective containers may be manufactured prior to the operator noticing a defect on the exterior surface of one or more of the sampled containers. In practice, these (completed) containers must be scrapped, resulting in significant cost to the container manufacturer.
Recently, automated systems have been developed and deployed in container production plants, wherein such systems are configured, through automated visual inspection, to detect defects on exterior surfaces of containers. These systems include multiple cameras that are positioned to capture images of an exterior surface of a container when the conveyor passes through an inspection region. The images captured by the cameras are then analyzed to determine whether the exterior surface of the container includes a defect. Many containers, such as aluminum beverage containers, however, have mirror-like qualities, rendering it difficult for automated inspection systems to differentiate between a reflected image (which is not a defect in the container) and an actual defect, such as a smear. More specifically, light that illuminates the exterior surfaces of a container may reflect off of numerous surfaces (including surfaces of adjacent containers on a conveyor that transports the containers through the inspection region), which causes reflections of portions of the adjacent containers to appear in images of the exterior surfaces of the container under inspection. Additionally, light can accentuate minor defects (that are tolerable to industry standards) on the external surfaces of containers. These reflections and accentuated minor defects render it difficult for an automated inspection system to distinguish among: 1) a reflection in an otherwise defect-free container; 2) an accentuated minor defect; and 3) a container that includes a defect.